Technology reduces cotton insect sprays

It’s a new day in cotton insect control, as each advancement in technology brings with it fewer required sprays for growers. A Georgia survey shows that insecticide applications went from an average of 15.8 per acre in 1986 to an average of three per acre in 2008.

“And the numbers are similar in Alabama and other states,” says Ron Smith, Auburn University Extension entomologist. “This is due primarily to the Boll Weevil Eradication Program and new technology.”

During the active phase of boll weevil eradication, cotton producers were making 12 to 15 insecticide applications per acre, says Smith. “Then we entered an era where we didn’t have weevils, but we had worms — especially resistant tobacco budworms — so we were still making a number of sprays. Then the Bt technology came in, and that reduces us down to the boll-feeding bugs, knocking our sprays down from about 1.3 to 3 per acre,” he says.

That’s where we are now, he adds, and things will change even more as cotton varieties advance from single-gene to multiple-gene technology.

“Some of our current sprays are for escaped bollworms on single-gene Bollgard cotton. We’ve traditionally had escapes since the technology was introduced in 1996. It’s a numbers game. When we have high pressure, there’s a certain percentage of these worms that escape from the Bollgard technology. The higher the pressure, the more escapes we see. It varies from year to year and from field to field,” says Smith.

When growers shift to Bollgard II and WideStrike varieties this year, these escapes will be reduced by about 10-fold, he says.

“We can’t say there still won’t be a need occasionally to over-spray, particularly for the July generation of the species. It’s going to be a little more likely in the WideStrike than in the Bollgard II, and whoever is monitoring the field will have to know which variety has been planted because the escapes will occur differently. The escapes in the Bollgard II won’t be as many as in Bollgard I, of course, but the ones that do occur will be associated with the white bloom — they will be in the zone of the plant where the white blooms are located. A scout could focus on that area and not waste a lot of time looking in other places,” says Smith.

On WideStrike cotton, the lowest concentration of the Bt gene is not in the white bloom, but in the terminals, he says. So escapes in those varieties will be in the top of the plant, and there will be no need in looking down lower.

“You need to be sure of the variety that is planted to make the most of your time. WideStrike is going to allow a few more escapes, but it’s still going to be perfect for the pyrethroid chemistry because it’s still active on the bollworm species. It’s the beginning of the stink bug phase of the season, and you’ll be knocking the front off of stink bugs, so it’s a perfect fit. Even if you don’t have high numbers of any of these pests, it serves the purpose of helping to set more fruit by working across several species,” says Smith.

Turning to fall armyworms, Smith notes that the 2x bollworm threshold has been used for 20 years now. “But this past year, we had the first heavy pressure we’ve seen since 1996 in many fields. In Alabama, there were some fields as far north as Shorter and Montgomery with high pressure. But the worst of the problem was in south Georgia, where some fields exceeded the threshold for six consecutive weeks, from mid-July to the first of September, and growers were spraying as much as they could with expensive chemistry.”

Based on experiences from last year, growers, in general, spent more money trying to control fall armyworms, says Smith. “The damage wasn’t that bad, but when you see those big worms lying in the blooms, it scares you. We were validated in using the 2x bollworm threshold. We’re wondering now if we might could go to an even higher threshold. The point here is that just because a farmer or scout sees a worm, they’re not all equal because they don’t all do the same damage.”

When cotton entomologists rated the effectiveness of different products on fall armyworms, the ratings, with five being the highest and one the lowest, were as follows, says Smith: Diamond — 3.86; Belt — 3.73; Steward — 2.94; and pyrethroids — 2.28. Bollgard was rated at 1.84 while Bollgard II was rated at 3.84 and WideStrike at 4.42.

“In WideStrike, the second gene is the armyworm gene, and that’s what makes it better.”

For stink bug control, it is now recommended growers use a “dynamic” threshold for control, says Smith. “This just means we recognize the shifts as we go through the season, and it depends on the number of bolls that are at risk. When you start off, there are very few bolls at risk. Then, each week, you move into a greater number. As cotton matures out, there’s a decrease in the threshold. This takes advantage of utilizing a lower number when you have fewer bolls at risk.”

The efficacy of the dynamic threshold has been proven in research conducted throughout the Southeast, he says. “Our benefits from spraying for stink bugs are coming from that three- to four-week period when most of the bolls are being set on the plant.”

When scouting for stink bugs, it’s recommended that the bolls be quarter-sized in diameter and soft, about 10 to 12 days old. The bolls should be soft enough so they can be crushed with the finger to help detect internal damage. Sample 20 to 50 bolls per field and use internal damage as shown in the dynamic threshold guidelines, says Smith. Also, correlate external to internal damage.

“We’re working with correlating the external to the internal damage because it’s much quicker to see the external. If we can correlate the external to the internal, we can breeze through a lot of bolls in a short amount of time — we need a faster system. Many cotton fields, especially in central and south Alabama, are being damaged by stink bugs from corn, says Smith. It appears stink bugs are moving from wheat, winter weeds or other hosts into corn and from corn into cotton, he says.

“From May to June and in early July, the only attractive crop making seed is corn. We think most of the stink bug populations are going to corn, and from corn, they’re spilling over into cotton, peanuts and soybeans for the remainder of the season. If we control stink bugs on corn, not only will the corn crop benefit, but we could also use it as a trap crop to help suppress the overall population. If you knock them back, it may be fall before they re-build.”